In conventional power management systems for turboprop engines, each engine is controlled by two levers: a speed lever and a power lever. The speed lever is coupled to a propeller governor. By adjusting the speed lever, a pilot can command the propeller governor to a desired speed. The propeller governor maintains the engine at the fixed speed by adjusting propeller blade angle. The power lever is coupled to a hydromechanical metering unit or digital engine control. In response to the angle of the power lever, the hydromechanical metering unit or digital engine control adjusts engine power by adjusting fuel flow or gas generator speed.
Much of the pilot's time is spent adjusting the engine to the required torque for specific operating conditions. Torque is a function of ambient conditions including temperature, altitude and mach number. Typically, the pilot looks at a flight manual for the torque for the day and adjusts the respective levers to obtain the power required for takeoff. The pilot sets the speed lever to a preset engine speed. Then, the pilot adjusts the power lever. As the power lever is advanced with the speed lever in a set position, the hydromechanical metering unit increases engine fuel flow within predetermined limits or the digital engine control increases gas generator speeds. For example these limits prevent compressor stall and combustor flameout. The power turbine is increased until the set point of the propeller governor is reached, at which time the propeller governor begins to adjust blade angle to hold the propeller at a fixed speed. Fuel flow is increased until the desired torque for the day is obtained, while blade angle is adjusted by the propeller governor to hold the propeller speed.
Once the aircraft has taken off and reached a cruising altitude, the pilot readjusts engine speed and torque to cruise levels. However, prior to adjusting the engine speed, the pilot reduces engine power to avoid overtorque or overtemperature conditions. Once engine power is low enough to safely adjust engine speed, the pilot iteratively adjusts the speed lever and observes an RPM gauge until the desired speed for cruise is reached. Then, engine torque is readjusted. Once again, the pilot refers to the flight manual to obtain the cruise torque at the prevalent ambient conditions. Then, he closes the loop on torque by iteratively adjusting the power lever.
As gas generator speed and/or fuel flow is being adjusted, the pilot must compensate for certain time constants associated with the engine. The steady-state torque level is not evident until the time constants associated with the propeller governor have washed out. Therefore, the pilot must close the loop on torque by watching a torque gauge and iteratively adjusting the power lever to obtain the desired torque at the ambient conditions.
Further, the pilot must compensate for changes in ambient conditions. As these conditions change, the desired torques also change. Unless the pilot adjusts the power lever, the engine is forced to operate at lower efficiency.
Changes in ambient conditions also affect the predictability of engine power. The engine requires more fuel to reach one hundred percent torque on a hot day than on a cold day. Therefore, on one day, the power lever angle required for one hundred percent torque might be different than the power lever angle on another day. As a consequence, the pilot cannot set the power lever to a known angle. Instead, the pilot must watch the torque gauge and iteratively adjust the power lever until the engine operates at one hundred percent torque.
The pilot must spend additional time adjusting the power lever during descent. Descent rate is directly affected by flight idle fuel flow provided by the hydromechanical metering unit. The flight idle fuel flow is preset achieve an optimal descent rate. However, at a fixed power lever angle, engine power varies as a function of altitude. Thus, the engine power changes as the aircraft descends and, as a result, the descent of the aircraft is not maintained at a constant rate. If a constant descent rate is required, the pilot must continuously adjust the power lever to maintain the requisite descent rate.
Finally, pilot workload is increased as a consequence of the engine power being unpredictable. Changes in ambient conditions affect the fuel flow required to obtain a given torque. For example, the engine requires more fuel flow to reach one-hundred percent power on a hot day than on a cold day. Therefore, the power lever angle required for one-hundred percent torque can vary from day to day. Because the pilot never knows the exact angle at which he can obtain one hundred percent power, he must iteratively adjust the power lever until such a power level is reached.
It is apparent that these procedures entail considerable pilot workload during critical phases of flight, including significant "heads down" activity.